The present invention relates to a device for and a method of fastening two component members in an injection-molded product or the like, and particularly relates to a device for and a method of fastening the component members by frictionally engaging protruded portions, such as screws or threads, provided on the component members.
It is common to use screws as a method of fastening various kinds of containers, particularly in the technical field in which two component members are fixed to each other such as when a lid is fastened to a container.
Generally in a screw type of fastening method the lead angle of each screw that performs the fastening is fixed. When this method is adopted in a lid of a container, fastening is performed by screws having a fixed lead angle that brings an interior ceiling surface of the lid and a planar surface at the opening of the container firmly in contact with each other. During the production of such a lid or a container, when the product is released from a core of a mold of injection molding, the core is rotated to eject the product, and therefore a screw having a fixed lead angle is used to facilitate mold-release.
A method for fastening two members having two kinds of screws with different lead angles is described in JP-A2002-34650, in which the moving speed of each member is made different in accordance with each kind of lead angle.
A device for fastening a bolt and a nut is described in JP-U-5-1010, wherein a left thread groove and a right thread groove with different lead angles are formed on one bolt and separate nuts having thread ridges that are screwed into the thread grooves with the corresponding lead angles of the bolt.
In a conventional fastening device, when screws are used as fixing means it is common for the lead angles of the screws to all be fixed. Consequently, a fastening force is obtained from the elastic deformation and frictional force of a male screw and a female screw, irrespective of the kind of material, and therefore a large rotation moment is required to obtain a strong fastening force. Accordingly, the fastening force is influenced by variations in the fastening rotational force of a lid on a container or the like, and there exists the problem that the lid may also be loosened easily.
With regard to the fastening rotational force of the lid, the rotational force applied after the interior ceiling surface of the lid and the planar surface opening of the container are in contact with each other contributes to the fastening force. Therefore, the frictional force of the lid's interior ceiling surface and the container opening's planar surface may result in a large loss of the lid's rotation force causing a wide variation in the fastening force.
With regard to fastening with screws having one kind of lead angle, a stopper is always needed at one end as in the relationship of a bolt and a nut and of a container and a lid, and fastening cannot be obtained in an optional place, thus limiting its use. Therefore, a fastening device with a bolt and a nut as described in JP-U-5-1010 is an alternative, but requires very special working and demands materials having a particular strength, which makes the device difficult to manufacture.
With regard to the fastening device with screws having different lead angles as described in JP-A2002-34650, two kinds of screws with different lead angles are disposed in parallel, which increases the handling convenience by having different moving speeds. However, this device is not intended for fastening itself.